Meaningful interpretation of U isotope measurements relies on unraveling the impact of reduction mechanisms on the isotopic fractionation. Here, the isotope fractionation of hexavalent U [U(VI)] was investigated during its reductive mineralization by magnetite to intermediate pentavalent U [U(V)] and ultimately tetravalent U [U(IV)]. As the reaction proceeded, the remaining aqueous phase U [containing U(VI) and U(V)] systematically carried light isotopes, whereas in the bicarbonate-extracted solution [containing U(VI) and U(V)], the δ238U values varied, especially when C/C0 approached 0. This variation was interpreted as reflecting the variable relative contribution of unreduced U(VI) (δ238U < 0‰) and bicarbonate-extractable U(V) (δ238U > 0‰). The solid remaining after bicarbonate extraction included unextractable U(V) and U(IV), for which the δ238U values consistently followed the same trend that started at 0.3-0.5‰ and decreased to ∼0‰. The impact of PIPES buffer on isotopic fractionation was attributed to the variable abundance of U(V) in the aqueous phase. A few extremely heavy bicarbonate-extracted δ238U values were due to mass-dependent fractionation resulting from several hypothesized mechanisms. The results suggest the preferential accumulation of the heavy isotope in the reduced species and the significant influence of U(V) on the overall isotopic fractionation, providing insight into the U isotope fractionation behavior during its abiotic reduction process.

Reductive immobilization of uranium has been explored as a remediation strategy for the U-contaminated subsurface. Via the in situ biostimulation of microbial processes, hexavalent U is reduced to less soluble tetravalent species, which are immobilized within the sediment. Although the mineral uraninite (UO2) was initially considered the dominant product of biological reduction, non-crystalline U(IV) species (NCU(IV)) are found to be abundant in the environment despite their greater susceptibility to oxidation and remobilization. However, it has been recently proposed that, through aging, NCU(IV) might transform into UO2, which would potentially enhance the stability of the reduced U pool. In this study, we performed column experiments to produce NCU(IV) species in natural sediment mimicking the environmental conditions during bioremediation. Bioreduced sediment retrieved from the columns and harboring NCU(IV) was incubated in static microcosms under anoxic conditions to allow the systematic monitoring of U coordination by X-ray absorption spectroscopy (XAS) over 12 months. XAS revealed that, under the investigated conditions, the speciation of U(IV) does not change over time. Thus, because NCU(IV) is the dominant species in the sediment, bioreduced U(IV) species remain vulnerable to oxidation and remobilization in the aqueous phase even after a 12-month aging period.

Uranium (U) in situ bioremediation has been investigated as a cost-effective strategy to tackle U contamination in the subsurface. While uraninite was believed to be the only product of bioreduction, numerous studies have revealed that noncrystalline U(IV) species (NCU(IV)) are dominant. This finding brings into question the effectiveness of bioremediation because NCU(IV) species are expected to be labile and susceptible to oxidation. Thus, understanding the stability of NCU(IV) in the environment is of crucial importance. Fe(II) minerals (such as FeS) are often associated with U(IV) in bioremediated or naturally reduced sediments. Their impact on the stability of NCU(IV) is not well understood. Here, we show that, at high dissolved oxygen concentrations, FeS accelerates NCU(IV) reoxidation. We hypothesize that either highly reactive ferric minerals or radical S species produced by the oxidation of FeS drive this rapid reoxidation of NCU(IV). Furthermore, we found evidence for the contribution of reactive oxygen species to NCU(IV) reoxidation. This work refines our understanding of the role of iron sulfide minerals in the stability of tetravalent uranium in the presence of oxygen in a field setting such as contaminated sites or uranium-bearing naturally reduced zones.

The release of arsenic (As) from deltaic sediments into groundwater is an issue that exposes millions of people in South-East Asia to toxic levels of As via drinking water. While reductive dissolution of iron oxyhydroxides and the associated As is a known mechanism of release, there are areas in which the sediments do not harbor iron oxyhydroxides. For instance, in the Vietnamese Mekong Delta, in a reduced, As-rich, peat layer harboring a brackish porewater and interpreted as a relic from a mangrove depositional environment [1], As is found associated with arsenian pyrite and natural organic matter (NOM). Flow-through experiments show that this peat layer releases a large amount of As and characterization of the solid phase reveals that both arsenian pyrite and NOM contribute approximately equally to the release. Furthermore, poisoning of the sediment decreased the amount of As released, suggesting a microbially-mediated process. However, the mechanism of As release from arsenian pyrite remained unclear. Further batch experiments revealed that pyrite was oxidized, releasing sulfate and As. The electron acceptor for the process is likely to be nitrate as it is the only detectable electron acceptor in the porewater of the peat layer (other than sulfate). Furthermore, amending peat with increasing concentrations of nitrate resulted in a linear and positive correlation with As released, supporting the role of nitrate. Similarly, it was suggested that heterotrophic denitrification could release As from NOM through oxidation of thiol groups. This work shows that a peat layer formed from a paleo-mangrove can serve as a source rather than a sink for As in situ through a microbial mechanism, provided that an appropriate electron acceptor is available. The implication is that the mobility of As in deltaic sediments is not solely dependent on reductive dissolution of iron oxyhydroxides but also on oxidative dissolution of arsenian pyrite and degradation of NOM.

Peat layers within alluvial sediments are considered effective arsenic (As) sinks under reducing conditions due to the binding of As(III) to thiol groups in natural organic matter (NOM) and the formation of As-bearing sulfide phases. However, their possible role as sources of As for anoxic groundwaters remains unexplored. Here, we perform laboratory experiments to provide evidence for the role of a sediment peat layer in releasing As. Our results show that the peat layer, deposited about 8,000 years ago in a paleomangrove environment in the nascent Mekong Delta, could be a source of As to porewater under reducing conditions. X-ray absorption spectroscopy (XAS) analysis of the peat confirmed that As was bound to NOM thiol groups and incorporated into pyrite. Nitrate was detected in peat layer porewater, and flow-through and batch experiments evidenced the release of As from NOM and pyrite in the presence of nitrate. Based on poisoning experiments, we propose that the microbially mediated oxidation of arsenic-rich pyrite and organic matter coupled to nitrate reduction releases arsenic from this peat. Although peat layers have been proposed as As sinks in earlier studies, we show here their potential to release depositional- and/or diagenetically-accumulated As.

In Switzerland, the Opalinus Clay formation is considered the most likely host rock for a deep geological repository for nuclear waste. In deep geological repositories, H-2 is expected to be the most abundant gas formed from the degradation of waste and from metal corrosion. The microbial community present in Opalinus Clay is capable of utilizing H-2 as an electron donor and sulfate as an electron acceptor to produce hydrogen sulfide. This could be problematic due to its potential for increasing the corrosion of metal waste canisters containing radioactive waste, however, the possible impacts of these processes on the clay rock have not been fully investigated. In this study, a series of microcosm experiments were set-up containing Opalinus Clay and porewater from the Mont Terri underground research laboratory (Switzerland) as an inoculum. Uninoculated microcosms were established to investigate abiotic processes. In the presence of clay, a higher aqueous sulfate concentration was detected than in those with only pore water present and this concentration decreased over time in the inoculated experiments. However, there was no evidence of hydrogen sulfide production in the aqueous phase. In all experiments with clay, there was an increase in aqueous Fe2+ concentrations with the highest concentrations found in uninoculated experiments. The sulfur speciation of the Opalinus Clay was analysed and the results of the inoculated sample suggested that hydrogen sulfide reacted with Fe2+, precipitating iron sulfide minerals. After the incubation period, the microbial community was dominated by the sulfate-reducing Desulfobulbaceae family. The study suggests that H-2-fuelled, microbially-mediated sulfate reduction can affect the mineral composition within the Opalinus Clay due to the precipitation of iron sulfide minerals. These precipitation reactions may enhance the long-term integrity of the repository by removing corrosive hydrogen sulfide from solution when sufficient Fe-2+ is available and so protecting the canisters containing the nuclear waste.

(*corresponding author: rizlan.bernier-latmani@epfl.ch) Bioreduction of hexavalent uranium [U(VI)] has been intensively investigated and proposed as a remediation strategy following uranium contamination in the subsurface. Initially, bioreduced uranium was believed to be solely uraninite [1], but later, other less crystalline species were documented [2]. It has also been shown that non-crystalline tetravalent U [U(IV)] species (NCU4) are the dominant products independently from the geochemical conditions and reductive pathway [3]. Because NCU4 appears to be more sensitive to reoxidation and remobilization to the aqueous phase [4], the use of bioreduction for remediation is now debated. This study investigates the stability of freshly bioreduced U(IV) under geochemical conditions that are relevant to the case of Rifle site in Colorado (USA). Furthermore, we document the potential for ageing processes to transform NCU4 to more crystalline phases and how this may affect the rate of NCU4 oxidation. For these purposes, uranium was reduced under both ironreducing and sulfate-reducing conditions in continuous flowthrough columns packed with Rifle aquifer background sediments. Artificial groundwater amended with uranium and multiple electron donors was the influent. The bioreduction phase was maintained for ~ 350 days until a significant amount of U was immobilized in the sediments. Prior to the re-oxidation phase, bioreduced sediments were characterized via X-ray fluorescence, X-ray absorption spectroscopy (XAS), and electron microscopy. The sediments harbouring sufficient amount of U(IV) were incubated in batch reactors under anoxic conditions and left to age for a minimum of 4 months to a maximum of 20 months. Fresh and aged samples were characterized with XAS to probe for evidence of mineralogical changes. In order to assess the stability of U(IV) species, bioreduced sediments were exposed to oxygen in flow-through reactors under various geochemical conditions. Preliminary results indicated that, in fresh sediments, the non-crystalline fraction of bioreduced U is most sensitive to oxidation.

Due to the limited efficiency of conventional biological treatment, innovative solutions are being explored to improve the removal of trace organic chemicals in wastewater. Controlling biomass exposure to growth substrate represents an appealing option for process optimization, as substrate availability likely impacts microbial activity, hence organic trace chemical removal. This study investigated the elimination of pharmaceuticals in pre-denitrifying moving bed biofilm reactors (MBBRs), where biofilm exposure to different organic substrate loading and composition was controlled by reactor staging. A three-stage MBBR and a single-stage reference MBBR (with the same operating volume and filling ratio) were operated under continuous-flow conditions (18 months). Two sets of batch experiments (day 100 and 471) were performed to quantify and compare pharmaceutical removal and denitrification kinetics in the different MBBRs. Experimental results revealed the possible influence of retransformation (e.g., from conjugated metabolites) and enantioselectivity on the removal of selected pharmaceuticals. In the second set of experiments, specific trends in denitrification and biotransformation kinetics were observed, with highest and lowest rates/rate constants in the first (S1) and the last (S3) staged sub-reactors, respectively. These observations were confirmed by removal efficiency data obtained during continuous-flow operation, with limited removal (<10%) of recalcitrant pharmaceuticals and highest removal in S1 within the three-stage MBBR. Notably, biotransformation rate constants obtained for non-recalcitrant pharmaceuticals correlated with mean specific denitrification rates, maximum specific growth rates and observed growth yield values. Overall, these findings suggest that: (i) the long-term exposure to tiered substrate accessibility in the three-stage configuration shaped the denitrification and biotransformation capacity of biofilms, with significant reduction under substrate limitation; (ii) biotransformation of pharmaceuticals may have occurred as a result of cometabolism by heterotrophic denitrifying bacteria.